Fetal Development and Multiple Gestations

Learning Outcomes

  • Distinctions:
    • Understand the difference between embryo and fetus.
  • Timeline of Fetal Development:
    • Familiarize with the gestational timeline (from fertilization to birth).
  • Monitoring Growth and Development:
    • Recognize how fetal development is monitored in antenatal care.
  • Calculating Estimated Delivery Date (EDD):
    • Learn to calculate the EDD based on the last menstrual period.
  • Types of Multiple Gestations:
    • Describe various forms of multiple pregnancies.

Timeline of Pregnancy

  • Key Terms:

    • Gestational Age: Duration post the first day of the last menstrual period (LMP).
    • Fetal Age: True age of the fetus from point of fertilization.

  • Breakdown of the 3 Trimesters:

    • 1st Trimester (Weeks 0-12): Embryonic stage to fetal stage transition.
    • 2nd Trimester (Weeks 13-26): Rapid development.
    • 3rd Trimester (Weeks 27-40): Final growth and preparation for birth.

Gestational Age vs. Fetal Age

  • Gestational Age: Measured from LMP, typically fetal age + 14 days.
  • Fetal/Embryonic Age: Measure from fertilization point.
  • Example: At 10 weeks gestation, the fetus is 8 weeks old.

Stages of Prenatal Development

  • Stages:
    1. Germinal Stage: Weeks 1-2
    2. Embryonic Stage: Weeks 3-8
    3. Fetal Stage: Weeks 9-birth.

First Trimester Details

  • Developmental Milestones:
    • Formation of features (arms, legs, etc.) by end of week 12.
    • By trimester end: 8 cm length; 58 g weight.
    • Risk of miscarriage drops significantly post 12 weeks.
  • Example: 80% of miscarriages occur within the first trimester.
  • 12-Week Scan: Key for determining EDD and checking fetal health.
    • Commonly around 10-14 weeks gestation.

Estimating Delivery Date (EDD)

  • EDD Calculation Steps:
    1. Identify the first day of LMP.
    2. Count back 3 months.
    3. Add 1 year and 7 days.
EDD Example Calculations
  • Example 1:
    • Last period: September 9, 2023 → EDD: June 15, 2024.
  • Example 2:
    • Last period: January 2, 2024 → EDD: October 8, 2024.

First Trimester Testing

  • Combined Test: Assess risk of Trisomy through ultrasound and blood tests.
    • Measures:
    • Crown Rump Length: 45-84 mm.
    • Nuchal Translucency (NT): Fluid measurement behind neck.
    • Biochemical markers: PAPP-A and Free β-hCG.

Second Trimester Highlights

  • Physical Changes:
    • Fatigue decreases; heartbeat audible at 15 weeks.
  • Fetal Movements: Quickening felt around weeks 17-20.
    • By trimester's end: 28 cm, 925 g.
  • 20-Week Scan: Anomaly scan for identifying 11 conditions (e.g., spina bifida, cleft lip).

Third Trimester Development

  • Growth Milestones:
    • Increases in body fat; active fetal movements.
    • Fetus prepares for delivery, typically turns head down by week 32.
  • Full Term Considerations:
    • Full term: Considered at 28 weeks, most deliveries between 40-41 weeks.
  • Average Birth Weights:
    • Males: 2.5 - 4.3 kg; Females: 2.4 - 4.2 kg.

Exploring Multiple Gestations

  • Types of Twins:
    • Monozygotic: Identical twins from one fertilized egg. (1/3 of twins)
    • Dizygotic: Fraternal twins from two fertilized eggs. (2/3 of twins)
  • Risks Involved: Different types of twins carry varying risks during gestation (e.g., Twins-Twin Transfusion Syndrome for monozygotic twins).
Differentiating Twin Types
  • Monozygotic Twins:
    • Shared placenta types: Monochorionic/monoamniotic; Monochorionic/diamniotic; Dichorionic/diamniotic.
  • Dizygotic Twins:
    • Always dichorionic/diamniotic; result from two separate fertilization events.